1. Technical Field
The invention relates to drivers, particularly to an electronic driver for providing a constant current source to multiple LED loads in parallel.
2. Related Art
Light emitting diodes feature long life and low power consumption and have become a primary of light source.
To illuminate LEDs, a driver is necessary. Usually, a plurality of LEDs are combined into a module as a load of the driver. In practice, a single driver is frequently used to drive multiple loads (LED modules) in parallel. Thus, a design target of the driver is to make all loads (LED modules) shine correspondingly. This target can be achieved by a constant voltage or current source.
The cheapest solution of a constant voltage driver is to add a current-limiting resistor connecting with an LED load in series and to provide a constant voltage source. However, such a current-limiting resistor will reduce the passing current. And the nonlinear V-I curve of the LED load cannot make the current stable enough. Additionally, the current passing the LEDs must vary when external voltage or forward current of LED changes. Suppose a current of 20 mA passes an LED when a rated forward voltage is 3.6V. If the voltage is changed into 3V (still in the allowable range of 3V to 4V) due to variation of temperature or modification of manufacture process, the forward current will drop to 14 mA. In other words, when the forward voltage is changed 11%, the forward current will vary 30%. Such a severe variation can change brightness of LEDs, which cannot be accepted by many apparatuses.
Because conventional constant current drivers are complicated in circuit and require so many components, it is hard to improve the yield rate and to reduce manufacture costs and volume. Additionally, when multiple LED loads in parallel are driven by a single driver and one of the loads is unexpectedly removed, the total current provided by the driver will be directly distributed among the remaining loads. This may damage the LEDs.